1. Field of the Invention
This invention pertains generally to electrical switching apparatus and, more particularly, to circuit interrupters including a trip unit. The invention also pertains to trip units for circuit interrupters. The invention further pertains to trip actuators for trip units.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices; circuit interrupters, such as circuit breakers; network protectors; contactors; motor starters; motor controllers; and other load controllers. Electrical switching apparatus such as circuit interrupters and, in particular, circuit breakers of the molded case variety, are well known in the art. See, for example, U.S. Pat. No. 5,341,191.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. Molded case circuit breakers typically include a pair of separable contacts per phase. The separable contacts may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip unit, which senses overcurrent conditions in an automatic mode of operation. Upon sensing an overcurrent condition, the trip unit trips the operating mechanism to a trip state, which moves the separable contacts to their open position. See, for example, U.S. Pat. Nos. 5,910,760; and 6,144,271.
U.S. Pat. No. 6,853,279 discloses a trip actuator including a bobbin assembly, a disk spacer, a disc magnet, which is preferably magnetized after certain assembly steps, a housing, a cover, a wave washer, an upper bushing, an armature or plunger, a lower bushing, an internal retaining ring, a spring and a set screw.
A known trip actuator consists of twelve parts, including an impregnated or coated set screw for spring adjustment, a brass bushing and a brass sleeve. The impregnated set screw is used to adjust spring compression and, therefore, trip force. The threads of the set screw are impregnated with a material that locks the set screw after it has been adjusted. However, it is believed that adjusting the screw might cause particles of the impregnated material in the threads to break free and potentially interfere with the operation of, and interface between, the bottom surface of the armature and the disc spacer. Hence, such debris might prevent proper magnetic seal force for the armature or plunger and, therefore, might cause magnetic shock out. As a result, the actuator force might be released prematurely due to mechanical vibration.
Hence, there is room for improvement in trip actuators for trip units.
There is also room for improvement in trip units including a trip actuator.
There is further room for improvement in electrical switching apparatus, such as circuit interrupters, including a trip unit having a trip actuator.